Method for manufacture stress detector

ABSTRACT

This invention relates to a method for manufacturing a highly reliable stress detector. Conventionally, a step of bonding and securing magnetic elements (4) to a driven shaft (1) receiving a stress was needed, so that a high technique was needed to secure the magnetic elements (4) uniformly at the entire surface and transversely symmetrically, disadvantageously increasing the process cost. 
     The present invention therefore has as its object the manufacture of a reliable stress detector by simplifying the process of securing the magnetic elements (4) and realizing a good attachment. According to this invention, a strip of magnetic thin sheet is secured around the driven shaft receiving a stress, and a selective coating removal treatment is applied to the magnetic thin sheet secured to the driven shaft to form magnetic elements parallelly arranged at a predetermined angle relative to the central axis of the driven shaft. Also, the present invention includes a technique using an epoxy-based bonding agent in the attachment and regulating its thickness. According to this invention, a highly reliable stress detector can be realized by a simple, low-cost process, and the stress detector of the present invention can be widely used in a wide field of industrial apparatus such as automobiles.

TECHNICAL FIELD

This invention relates to a method for manufacturing a stress detector,which is used in manufacturing a torque detector or the like for thenon-contact measuring of the axle torque on the driven shaft such as arotary shaft widely used in the industrial equipments such as anautomotive steering shaft, a propeller shaft, an axle as well aselectric motors and compressors.

BACKGROUND ART

FIG. 1 is a schematic diagram showing a torque detector disclosed inJapanese Patent Laid-Open No. 61-178627 for example, in which figure,(1) is a driven shaft to which a torque is applied, (3) are coils fordetecting the amount of change in magnetic permeability, (4) aremagnetic elements of which magnetic permeability change in accordancewith the amount of the internal strain generated by the torque appliedto the driven shaft (1). A plurality of magnetic elements in the form ofstrip pieces cut from a thin sheet of magnetic material are arranged insymmetry around the driven shaft (1) at an angle of ±45° for example.

The operation will now be described When a Lorque is applied from theexterior to the driven shaft (1), a main stress having a main axis alongthe longitudinal direction of a group of the rectangular magneticelements is generated. When it is assumed that this stress is a tensileforce in the magnetic element group on the left side of the driven shaft(1) as viewed in FIG. 1, then the stress on the right side element groupis a compression force. In general, when a stress is applied to amagnetic material of which constant of magneto-restriction is not zero,its magnetic property varies and accordingly the magnetic permeabilitychanges as well known. This phenomenon is utilized in amagnetoristriction transducer for converting mechanical energy intoelectrical energy, and corresponds to Villari effect according to whichthe magnetic permeability of a magnetic material varies in accordancewith the amount of deforming of the material. Also, when themagnetoristriction constant, which is an amount quantitably indicatingthe amount of magnetoristriction, is positive, the magnetic permeabilityincreases when a tensile force is applied, and the magnetic permeabilitydecreases when a compressive force is applied. It is also known that aquite opposite result is obtained when the constant ofmagnetoristriction is negative. Therefore, since the magneticpermeability of the magnetic elements (4) changes as they deform inaccordance with the amount of the externally applied torque, the amountof the torque applied to the driven shaft (1) can be determined bydetecting the change in magnetic permeability as the change in magneticimpedance by the sensor coils (3) wound around the driven shaft (1).

As a method for securely attaching the magnetic elements (4) on thedriven shaft (1) of the torque detector of the above-describedconstruction, a method in which the magnetic elements are formed from athin sheet of a magnetic material and the magnetic elements (4) arebonded one by one to the driven shaft (1) at a predetermined anglethereto, or a method in which the magnetic elements (4) are bonded to anonmagnetic film by a bonding agent or the like and then this film iswound around the driven shaft (1) and secured thereto by an epoxythermo-setting bonding agent or the like.

FIG. 2 is a flow chart showing where the magnetic elements are bonded bya bonding agent onto a conventional non magnetic film. As seen from thefigure, a strip of thin magnetic sheet material is cut into a pluralityof magnetic elements (Step 41). Then, a bonding agent is applied on theplastic film (Step 42). The magnetic elements are placed on the bondingagent applied to the plastic film (Step 43), and the plastic film iswound around the driven shaft (1) (Step 44). The plastic film is securedby caulking at its periphery and thermally set to firmly attach themagnetic elements (4) around the driven shaft (1) (Step 45).

According to the conventional manufacturing method, a careful attentionmust be paid to arrange the magnetic elements at a predetermined anglewith respect to the driven shaft (1). Particularly, when the plasticfilm is bonded by a thermo-setting bonding agent after the magneticelements are bonded to the plastic film, it is necessary that theplastic film be of a heat-resistive material such as polyimide, Teflon,polyester/polyimide and that the bonded portion be firmly secured,allowing no movement. Particularly, when an amorphous magnetic materialis used as the magnetic sheet material, a large external force isnecessary to firmly and intimately secure them around the driven shaft(1) since the amorphous magnetic material is very hard and elastic.Also, the efficiency of the transmission of the stress due to the torquebetween the driven shaft (1) and the magnetic elements (4), that is, themagnitude of the stress propagation in the radial direction within thebonding agent layer depends upon the thickness of the bonding agentlayer, so that it is desirable that the thickness of the bonding agentlayer is thin and uniform in order to increase the sensitivity to thetorque. In particular, when the magnetic elements are arranged in pairsin differential-type as shown in FIG. 1, the imbalance of the thicknessof the bonding agent layer on the left and right directly affects thebalance of the output, so that it is necessary to pay attention toensure that the external force applied for securing as well as theamount of the applied bonding agent is equal between both left and rightsides and is uniform throughout.

Particularly, when an imbalance appears in the thickness of the layer ofthe bonding agent on the left and right sides, this becomes an offset inthe static characteristics of the sensor, which offset is difficult tobe temperature-compensated because the offset amount increases anddecreases according to temperature and is not an amount that can beestimated at the time of manufacture.

Since the conventional method for manufacturing a stress detector is asabove described, the process conditions at the time of bonding must besufficiently suppressed and the attaching conditions for the magneticelements (4) must be made uniform throughout and symmetry in theleft-and-right wise, so that a very reliable, delicate and complextechnique is needed. Even with such technique, it is difficult to obtaina quality stress detector, posing problems of time-consuming adjustment,high process costs, etc.

This invention has been made in order to solve the above problems andhas as its object the provision of a method for manufacturing a highlyreliable stress detector in which the process for securing the magneticelements (4) can be simplified, and the securing conditions can beuniform throughout and symmetric in transverse direction, and thedispersion of the detector quality can be decreased.

SUMMARY OF THE INVENTION

The method for manufacturing a stress detector of the present inventioncomprises the steps of securing a strip of a magnetic thin sheet arounda driven shaft, and applying selective coating eliminating treatment tothe magnetic thin sheet secured to the driven shaft to obtain magneticelements arranged at a predetermined angle with respect to the centralaxis of the driven shaft.

In the step of securing the magnetic thin sheet to the driven shaft,when a bonding agent is to be applied to the magnetio thin sheet, sincethe area of the magnetic thin sheet is considerably large as compared tothe conventional magnetic element, it is easy to apply a bonding agentto the magnetic thin sheet, so that the bonding agent can be relativelyeasily uniformly applied. In the step of securing the strip of themagnetic thin sheet, the axial dimensional accuracy of the driven shaft(1) is not required, and when the selective removal treatment is appliedafter securing, not only the accuracy of the axial dimension, but alsoall the positional accuracy in the circumferential direction, width ofthe magnetic elements, gap between them, etc. can be obtained within thetolerance of the positional error of the selective removal treatment.Further, when the magnetic thin sheet is to be secured by externallyutilizing a caulking tool or the like to increase the intimate contactrelationship, uniform application of a pressure over the entire surfacethroughout is easy. In particular, when the arrangement is of thedifferential type in which the magnetic elements are in pairs, the leftand the right elements can be formed at the same time by a singleprocess, enabling to easily obtain the arrangement which is uniform overthe entire surface and symmetry in the left-and-right wise direction.

Also, when end portions of the pre-formed strip of the magnetic thinsheet are diagonally cut to have a predetermined angle with respect tothe drive shaft, no discontinuity is generated in a plurality ofmagnetic elements formed by the selective removal treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a general torque detector as oneexample of a stress detector; FIG. 2 is a flow chart showing theconventional method for manufacturing a stress detector; FIG. 3 is aflow chart showing one embodiment of the present invention; FIGS. 4(a)and 4(b) are perspective views showing the stress detector of oneembodiment of the present invention; FIG. 5 is a flow chart showinganother embodiment of the present invention; FIGS. 6(a) and 6(b) areperspective views of the stress detectors according to other embodimentsof the present invention; FIG. 7 is a flow chart showing anotherembodiment of the present invention; FIG. 8 is a characteristic diagramshowing the relationship between the thickness of the bonding agent andthe shear strength; FIG. 9 is a characteristic diagram showing therelationship between the thickness of the bonding agent and the stresspropagation ratio; and FIGS. 10, 11(a) and 11(b) are schematic diagramsillustrating another embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

One embodiment of the present invention will now be described inconjunction with the drawings. Although the torque detector of FIG. 1 isshown as being of the differential type in which the magnetic elementgroup is parallel arranged in parallel at an angle of 45°, themanufacture of only one of them will be described. FIG. 3 is a flowchart showing the steps of the method for manufacturing a stressdetector according to one embodiment of this invention, and FIG. 4 is aperspective view showing the stress detector being manufactured. In Step11, a strip of magnetic thin sheet (2) made of an amorphous magneticmaterial having a large magnetoristrictive effect such as Fe, Ni and Cobased metals and the end portions are diagonally cut so that they aresubstantially at equal angles to the angle of the magnetic elements (4)relative to the axis when the thin sheet is secured to the driven shaft(1). Then, in Step (12), an epoxy resin based thermo-setting bondingagent for example is applied to the entire surface of the magnetic thinsheet (2). This is closely wound around the driven shaft (1) as shown inFIG. 4(a) (Step 13) and the periphery is secured by applying thereto adifferential pressure more than about 1 kgf cm² by such as a caulkingtool. Then, this is placed into an oven to heat and cure at about 120°C. to secure around the driven shaft (1) (Step 14). After heating andcuring, a selective coating removal treatment generally known asphotoetching treatment is applied, whereby the parallel magneticelements (4) at an angle of 45° for example with respect to the centralaxis of the driven shaft (1) can be obtained as shown in FIG. 4(b). Thebasic process of the photoetching treatment includes, as shown in FIG.3, cleaning of the surface, applying and drying of photo-resist at Step15, exposing, developing and baking of the photo-resist in Step 16, andapplying selective coating removal treatment by etching, removing,surface cleaning of the photo-resist as shown in Step 17.

The strip of the magnetic thin sheet (2) in this example has a lengthcorresponding to the circumference of the driven shaft (1}that isshorter than the circumference of the driven shaft (1) and has an axialdimension that is longer than the axial length of the magnetic elements(4) to be formed, so that the magnetic thin sheet can be easily wound onthe driven shaft (1) in Step 13. Further, the circumferential endportions are formed to be at an angle substantially equal to theinclined angle of the magnetic elements (4) with respect to the axiswhen the thin sheet is attached to the driven shaft (1). Therefore, nomagnetic element (4) has joint. If, for example, the end portions arenot diagonally cut, the magnetic element (4) at the end of the turnsinevitably has joint. When a laser beam source is used to make a slantededge at a predetermined angle with respect to the driven shaft (1), theedge can be sharply cut, but other cutting means can be used. Also sincethe magnetic thin sheet (2) is made of an amorphous magnetic material,rough surface of this material can be used to secure it to the drivenshaft (1) in Step (13), resulting in an easy attachment owing to thesurface conditions. Also, when the strip of the amorphous magneticmaterial is to be cut at its end portions in Step (11) the amorphousmagnetic material may be irradiated by the laser beam source at therough surface, the dimensional accuracy of the cut edge can be madehigher because the rough surface is superior in light absorbingcharacteristic.

In this embodiment, the bonding agent is applied over substantiallyentire surface of the strip of the magnetic thin sheet (2) and thesurface area of the magnetic thin sheet (2) is relatively large ascompared to that of the conventional magnetic elements (4), so that theuniform and symmetrical application of the bonding agent can easilY bemade. Also, as to the positional accuracy of the plurality of themagnetic elements (4) thus obtained, a high accuracy is not requiredwhen the magnetic thin sheet (2) is to be secured on the driven shaft(1), and an accuracy within a set tolerance associated with theselective coating removal treatment such as mask alignment.

Another embodiment of the present invention will now be described inconjunction with figures. FIG. 5 is a flow chart showing the steps ofthe method for manufacturing the stress detector according to theembodiment, and FIG. 6 is a perspective view showing the stress detectorbeing manufactured. First, as shown in Step (21), a magnetorestrictivematerial such as pure Ni or Permalloy or the like is plated on thesurface of the driven shaft (1), and the magnetic thin sheet (2) issecured around the driven shaft (1) as shown in FIG. 6(a).

Thereafter, similarly to the above embodiment, the plurality ofparallelly arranged magnetic elements (4) defining a predetermined anglerelative to the driven shaft (1) by the so-called photoetching treatmentstep in Steps (22), (23) and (24) (FIG. 6(b)).

In this embodiment, homogenious coating can be easily obtained by makingsure that conditions in the plating tank such as the electric fielddistribution and temperature distribution are uniform by agitating orrotating. As to the positional accuracy of the plurality of the magneticelements (4), advantageous results similar to those obtained in theabove embodiment can be obtained.

A still another embodiment of the present invention will now bedescribed in conjunction with the drawings. FIG. 7 is a flow chartshowing the steps of the method for manufacturing the stress detector ofthis embodiment in sequence. A bonding agent is applied (Step 32) to thestrip of the magnetic thin sheet (2) of which end portions arediagonally cut in Step (31). Then, this is wound to the driven shaft (1)(Step 33) and heated ant cured at about 120° C. to securely attachedaround the driven shaft (1) (Step After the thermosetting, unnecessaryportions are selectively removed through the use of a laser source suchas YAG, CO₂, etc. of more than 10 W, thereby obtaining a plurality ofparallelly arranged magnetic elements (4) (Step (35)).

In this case, it is necessary to make sure that the laser source used beof high power to ensure that the trimming completes before the entiremagnetic thin sheet (2) is heated and that the temperature of the bondedportion become unnecessarily high.

Although the laser trimming is employed in this embodiment, it is notlimited to the laser trimming, but other physical method using plasmaetching, ion etching, electron beam irradiation or the like can be usedto obtain similar advantageous results.

While, in the above embodiment, the description has been made as to themethod for manufacturing the magnetic elements (4) parallelly arrangedin an inclined relationship in one direction with respect to the centralaxis of the driven shaft (1), it is not limited to this but isapplicable to the differential type detector as shown in FIG. 1, forexample. In this case, the circumferential end portions of the strip ofthe magnetic thin sheet (2) must be cut in a V-shape.

Also, while, in the above embodiment, the manufacturing method of thetorque detector has been explained, the external force is not limited totorque, and it can be applied generally to detectors utilizing thechange in magnetic permeability due to the internal stress in themagnetic elements (4) induced by an external force, providing similaradvantageous results in load meters, position detectors, pressuregauges, etc.

As has been described, according to the present invention, a method formanufacturing a stress detector can be provided, which comprises thesteps of securing a strip of a magnetic thin sheet around a drivenshaft, and applying selective coating eliminating treatment to themagnetic thin sheet secured to the driven shaft to obtain magneticelements arranged at a predetermined angle with respect to the centralaxis of the driven shaft, whereby a highly reliable stress detector inwhich the manufacturing process can be simplified with high accuracy andsmall dispersion.

The description will now be made as to the first to the thirdembodiments in connection with the method for controlling the thicknessof the bonding agent layer in the embodiment shown in FIG. 3.

The first embodiment will now be described in conjunction with thefigure. The structure of the torque detector according to thisembodiment is similar to that shown in FIG. 1, but the magnetic thinsheet (2) is an amorphous magnetorestrictive material (for example,Product name 2826 MB of Allied Chemical Co.), and the bonding agent forbonding the magnetic thin sheet (2) to the driven shaft (1) is athermosetting epoxy-based film bonding agent having a thickness of lessthan 100 μm (for example, Product name AF147, Sumitomo-3M Co.). Theoperation as the torque detector is similar to that of the conventionaldesign.

FIG. 8 illustrates the relationship between the thickness [μm] of thebonding agent and its shearing strength [MPa], from which it is seenthat the shearing strength is maximum when the thickness of the bondingagent is in the order of 50 μm, a desirable shearing strength andtherefore a desirable bonding strength can be obtained when thethickness is within a range of from 20 μm to 100 μm.

Further, with the Young's modulus E₁, the linear expansion coefficientβ₁ and the thickness t₁ of the driven shaft (1), the rigidity factor Gcand the thickness h of the bonding agent, and the each length or theschevron length L, Young's modulus E₂, the linear expansion coefficientβ₂ and the thickness t₂ of the magnetic thin sheet (2), then, since theYoung's modulus E₁, E₂ of the driven shaft (1) and the magnetic thinsheet (2) are larger than the Young's module of the bonding agent by theorder of two, it is assumed that the compression/tension stress parallelto the surface of the driven shaft (1) is absorbed by the sheardeformation of the bonding agent, that only tensile or compressivedeformation in the longitudinal direction is generated in the drivenshaft (1) and the magnetic thin sheet (2) and that no bendingdeformation is generated. Also, when it is assumed that a deformation u₁in the x direction (longitudinal direction) of the driven shaft (1) anda deformation u₂ of the magnetic thin sheet (2), a shear deformation τin the bonding agent is in the following relationship when the bondingagent has the rigidity factor Gc and the shearing stress: ##EQU1##

Also, since t₂ is of the order of 25 μm and t₁ is several thousands μm,an assumption of E₁ t₁ >E₂ t₂ is held so that the ratio of stresstransmitted from the driven shaft (1) to the magnetic thin sheet (2)through the bonding agent or the stress transmission ratio σ₂ x/σ₂ o is:##EQU2## where, A=Gc/hE₂ t₂. FIG. 9 shows a relationship between thethickness h of the bonding agent and the stress transmission ratioagainst the parameter of the length L of the magnetic thin sheet (2),the stress transmission ratio being changed as the change in thicknessof the bonding agent. In this embodiment, the thickness of the bondingagent is equal to or more than 20 μm and equal to or less than 100 μm,so that the stress transmission ratio is large, providing a stresstransmission ratio equal to or more than 70% even when the schevronlength of the magnetic thin film (2) is 2 mm for example, increasing thetorque detecting sensitivity.

Also, since the efficiency is increased by the improvement in the torquedetecting sensitivity, the applied current to the detection coil (3) canbe made small-sized, decreasing the heat generation in the detectioncoil (3) and the transistors, capacitors, resistors and the like in thedetection circuit (not shown), so that the power-on-drift and aging aredecreased and the reliability is increased.

As has been described, according to this embodiment, the elongatedmagnetic elements (4) are bonded to the outer circumference of thedriven shaft by the epoxy-based bonding agent having a thickness equalto or more than 20 μm and equal to or less than 100 μm and the thicknessof the bonding agent is small, so that the stress transmission from thedriven shaft to the magnetic elements (4) is increased to improve thetorque detection sensitivity. Also, since the bonding agent is thin, thebonding strength is increased, resulting in an increased reliability.Further, since the current to the detection coil can be made smallbecause the efficiency is increased due to the increase in the detectionsensitivity, the heat generation is decreased and the operating life canbe increased.

The second embodiment will now be described in conjunction with thedrawings. FIG. 10 is a sectional view showing the main portion of thestrain detector according to the second embodiment, wherein 40designates a bonding agent in which a large number of same-diameternon-magnetic balls 41 are mixed. The non-magnetic balls 41 are aluminaballs of a diameter equal to or less than 100 μm, and these balls 41 aremixed with the bonding agent 40 at a proportion of 5 weight %. Mixing iscarried out by agitating in vacuum to eliminate any bubbles. As for thebonding agent 40 a thermosetting single-liquid epoxy-based bonding agentis used. Also, as for the magnetic thin sheet (2), an amorphous magneticmaterial having Young's module of 16500 kgf/mm² is used. The magneticthin sheet (2) is bonded and secured to the outer circumference of thedriven shaft 1 by the bonding agent 40 in which the nonmagnetic balls 41are mixed.

In the above construction, the reason for the nonmagnetic balls 41 beingused is that if magnetic balls are used they serve as passages for themagnetic flux generated from the detection coil 3, decreasing the numberof intersecting magnetic fluxes of the magnetic thin sheet (2) anddecreasing the sensitivity. Also, the reason for the amorphous magneticmaterial is used is that the magnetoristriction constant is large andthat the stress concentration is difficult to occur because it is hardand not easily deformed. Also the non-magnetic balls 41 are mixed intothe bonding agent 40 by agitating in vacuum because bubbles may beformed and become too hard to sufficiently agitate if mixing is carriedout in air.

In the above-described strain detector, the thickness of the bondingagent 40 becomes substantially equal to the diameter of the non-magneticballs 41 to become uniform and the error is within ± several %.Therefore, as shown in FIG. 1, the stress transmitted from the drivenshaft 1 to the magnetic elements (4) is uniform, providing a linearstable output characteristic. Also, no dispersion is observed intemperature characteristic and it is cancelled out when the magneticelements (4) are used in differential manner. Further, since thenon-magnetic balls 41 are mixed into the bonding agent 40, the entirehardness of the bonding agent layer is increased, making the stresstransmission ratio higher to increase the sensitivity and since thecoefficient of linear expansion β of the bonding agent layer becomesmall, decreasing the difference in the coefficient of linear expansionβ relative to the driven shaft 1 and the magnetic elements (4), wherebythe thermal stress decreases. Also, since the efficiency is improved dueto increase in the sensitivity, the applied current to the detectioncoil 3 can be small, the heat generation in the circuit portion isdecreased and the aging is slow.

As for the non-magnetic balls 41, glass beads, acrylic/polyethylenebeads or the like may be used.

As has been described according to this embodiment, the magnetic thinsheet (2) is secured to the driven shaft by the bonding agent includingthe non-magnetic balls, the thickness of the bonding agent is determinedby the diameter of the non-magnetic balls and is uniform, and the stresstransmitted from the driven shaft 1 to the magnetic elements (4) throughthe bonding agent is uniform. Therefore, the output characteristic andtemperature characteristic become good and the sensitivity is improved.Further, the bonding agent becomes to have a high sensitivity and asmall coefficient of expansion, and also have a high stress transmissionratio and small residual thermal stress, increasing the sensitivity.

The third embodiment will now be described.

The third embodiment will be described in conjunction with the drawing.In this embodiment, the magnetic thin sheet 2 is bonded to the drivenshaft 1 through the film-shaped bonding agent 43 as shown in FIG. 11(a).The film-shaped bonding agent 43 is an epoxy resin 40 impregnated into anon-magnetic net-shaped member 42 as shown in FIG. 11(b). Therefore, themagnetic thin sheet (2) is wound around the driven shaft 1 with the filmshaped bonding agent 43 applied on the magnetic thin sheet (2) or thedriven shaft 1, so that these two are bonded together through the filmshaped bonding agent 43. Thereafter, the magnetic thin sheet (2) ispressed to the driven shaft 1 by the auto-crape process or pressurizeprocess similar to the conventional method, and the film-shaped bondingagent 43 is heated and cured under this condition. Finally, the magneticthin sheet (2) is subjected to the selective removal treatment byetching, thereby forming the schevron-shaped magnetic layers.

In this embodiment, since the bonding agent 43 is pre-formed in theshape of a film, it is not necessary to apply by painting and onlynecessary to attach to the magnetic thin sheet (2) and the drivenshaft 1. Therefore, no uneven application and no bubbles are notinvolved, increasing the bonding strength and sensitivity due to theincreased bonding area. Also, since the net-shaped member serves as aspacer, the thickness of the bonding agent 43 can be uniform (less than±5%), and the output characteristic as well as the temperaturecharacteristic is uniform.

When an epoxy resin or the like is applied to both of the driven shaft 1and the magnetic thin sheet 2 before the bonding agent 43 is attached(plasma treatment), the bonding can be smoothly carried out. Also, thenet-shaped member 42 is made non-magnetic in order to prevent that themagnetic flux generated by the current flowing through the detectioncoil 3 flows through the bonding agent 43 and that the magnetic fluxflowing through the magnetic thin sheet 2 is decreased and thesensitivity is lowered.

As has been described according to the present invention, the bondingagent needs not to be applied by painting to the magnetic layer, so thatno uneven application of the bonding agent and no bubble is involved andthe bonding area is large, whereby the bonding strength is high and thereliability is high. Also, since the transmission of the stress from thedriven shaft to the magnetic layer is efficient, the detectionsensitivity is improved. Also, since the net-shaped member of thebonding agent serves as a spacer, the thickness of the bonding agent isuniform, improving the temperature characteristic of the residualthermal stress or the like and the output characteristic linearity.

We claim:
 1. A method for manufacturing a stress detector comprising thesteps of securing a strip of magnetic thin sheet around the driven shaftreceiving a stress, and applying a selective coating removal treatmentto said magnetic thin sheet secured to said driven shaft to formmagnetic elements parallelly arranged at a predetermined angle relativeto the central axis of said driven shaft.
 2. A method for manufacturinga stress detector as claimed in claim 1, wherein said selective coatingremoval treatment is photo-etching treatment.
 3. A method formanufacturing a stress detector as claimed in claim 1, wherein saidselective coating removal treatment is one of plasma-etching,ion-etching, laser trimming and electron beam irradiation.
 4. A methodfor manufacturing a stress detector as claimed in one of claims 1 to 3,said strip of magnetic thin sheet is formed by selecting the lengthcorresponding to the circumference of said driven shaft to be shorterthan the circumferential length of said driven shaft, and the axiallength to be longer than the axial length of said magnetic elements tobe formed, and forming the circumferential end portions at an anglesubstantially equal to the angle of inclination of said magneticelements when it is secured to said driven shaft.
 5. A method formanufacturing a stress detector as claimed in claim 1, saidcircumferential end portions of said magnetic thin sheet is cut by alaser source at a predetermined inclined angle relative to said drivenshaft.
 6. A method for manufacturing a stress detector as claimed in oneof claims 1, 2, 3 or 5, wherein said magnetic thin sheet is an amorphousmagnetic material, and said amorphous magnetic material is secured atits surface having recesses and projections to said driven shaftsurface.
 7. A method for manufacturing a stress detector as claimed inclaim 5, wherein said magnetic thin sheet is an amorphous magneticmaterial, and said amorphous magnetic material is cut by irradiating alaser beam from a surface having recesses and projections.
 8. A methodfor manufacturing a stress detector as claimed in claim 1, wherein saidsecuring step includes using an epoxy-based bonding agent equal to andmore than 20 μm and equal to and less than 100 μm.
 9. A method formanufacturing a stress detector as claimed in claim 8, wherein saidepoxy-based bonding agent has mixed therein a large number ofsame-diameter non magnetic balls.
 10. A method for manufacturing astress detector as claimed in claim 8, wherein said epoxy-based bondingagent is a film-shaped bonding agent in which a resin is impregnatedinto a net-shaped member.